硼向奥氏体晶界的非平衡偏聚

采用高分辨率的径迹显微照相技术,研究了淬火硼钢中硼向奥氏体晶界偏聚的规律,定量地测定出跨过奥氏体晶界的硼的成分剖面图以及非平衡晶界偏聚的特征参量（晶界贫硼区宽度、晶界富集程度和富集带宽度）。试验表明,这种偏聚具有如下特征:
在偏聚晶界的二侧存在有一定宽度的贫硼区,晶界偏聚的硼,是在冷却过程中由该区富集而来;这种偏聚对冷却速度很敏感,急速冷却可加以抑制,随冷却速度降低,晶界偏聚由连续的偏聚带,逐步发展为不连续的聚集直至明显的硼相析出,贫硼区宽度与冷却速度的平方根成反比;它的温度关系与平衡晶界偏聚的予吉相反,随淬火温度升高,晶界偏聚程度与贫硼区宽度增加。
试验论证了,淬火硼钢中硼向奥氏体晶界的偏聚,与平衡晶界偏聚有明显区别,它是在冷却过程中发生的一种非平衡的晶界偏聚。     

超低碳贝氏体钢快速加热奥氏体长大过程中移动晶界上硼的反常偏聚

利用径迹显微照相技术研究了超低碳贝氏体钢焊接热影响区在焊接热循环快速加热过程中硼在奥氏体晶界上的偏聚行为。发现以高密度位贝氏体为原始组织的材料进行快速加热时，新形成的奥氏体晶粒边界上在很高温度下仍会出现反常的晶界硼偏聚。用晶界位错驰原制对这种新的非平衡现象进行了讨论。     

硼钢淬透性与硼偏聚

用改进的径迹显微照相技术研究了50B钢800—1150℃顶端淬火试样中硼的分布状态及其变化规律与硼钢淬透性的关系。试验表明,硼钢淬透性的变化与冷却时硼向奥氏体晶界偏聚的发展速度密切相关。晶界硼偏聚来自二侧贫硼区,利用成分剖面图技术可定量测定这种偏聚的发展速度。
晶界硼偏聚发展过快与过慢均对发挥硼效应不利。试验表明,只有当晶界硼偏聚的发展速度与基体成分相变孕育期能较好配合时,硼的效应最大。利用这种观点可正确说明硼钢淬透性的变化特征及认识与分析影响硼钢淬透性的各种因素。     

硼向奥氏体晶界非平衡偏聚的机制

业已证实,淬火硼钢中硼向奥氏体晶界的偏聚,是在冷却过程中发生的一种非平衡的晶界偏聚。本文通过解变温扩散方程,导出了非平衡晶界偏聚的理论公式,建立了晶界贫硼区宽度与淬火加热温度、冷却速度以及非平衡晶界偏聚扩散激活能与扩散常数之间的关系。理论予言与实验结果很好地吻合。
根据实验结果和理论分析,提出这种非平衡晶界偏聚的机制,是在冷却过程中,过饱和空位或双空位带着硼原子向晶界（空位阱）迁移的结果。
基于这种非平衡晶界偏聚的新概念,可以较完满地说明影响硼钢淬透性的众多复杂因素。     

溶质原子晶界偏聚动力学过程的数值模拟

建立了溶质原子在晶界的平衡偏聚、非平衡偏聚、晶界偏聚溶质向沉淀析出转化以及冷却速度等因素的晶界偏聚物理模型和数学模型.模型考虑了晶界及晶界附近扩展畸变区对溶质的吸附作用和吸附能力.对含硼0.0010%的Fe-40%Ni-B合金体系从1150℃连续冷却到640℃的过程中硼的晶界偏聚状态进行了模拟计算.计算表明,晶界区域硼富集因子在降温初期增加较快,随后增幅变缓,模拟数据显示过程中有晶界区域硼原子向晶内的反向扩散;当晶界上偏聚的硼转化为析出物时,晶界区域富集因子的增加再次变快.模拟计算结果与已发表的实验结果吻合较好.     

微量硼对低碳贝氏体钢过冷奥氏体转变的影响

采用Gleeble-3800热模拟实验机对加入微量硼的低碳钢进行了动态CCT曲线的测定和组织分析,研究了钢中微量硼在奥氏体晶界上的偏聚作用。结果表明,微量硼的加入降低了碳原子在晶界上的吸附趋势,导致了晶界处珠光体形核率的下降,改变了动态CCT曲线的形状,抑制了珠光体的转变,能明显提高钢的淬透性和强韧性。     

Fe-3%Si中再结晶运动晶界上硼的偏聚

利用硼径迹显微照相技术研究了在Fe-3%Si合金中热变形再结晶运动晶界上硼的偏聚行为和特点。结果表明;在1000℃、20%变形后再结晶过程中,硼在运动晶界上存在明显的富集,而当晶界运动停止时,这种富集也将随之消失。利用位错在晶界消失的驰豫机制,对这种溶质原子在移动晶界上的非平衡偏聚作了初步的探讨。     

含铜超低碳微合金钢冷却过程硼的晶界偏聚

用硼径迹显微照相技术研究含铜超低碳微合金钢从1 150℃以5℃/s冷却到850℃过程中硼的晶界偏聚的形成与发展的过程。统计分析了冷却到不同温度时富集因子与贫化区宽度的变化,研究表明在冷却初始阶段硼—空位复合体迅速扩散到晶界上,使晶界上的硼迅速增加,在1 090℃达到极大值,冷却到1 090～940℃区间时,随着温度的降低晶界上的硼发生明显的反向扩散,晶界上的硼逐渐减少,继续降低温度,随着析出物的出现反向扩散变弱,晶界上硼的偏聚量又开始增加。     

冷却过程中硼的晶界非平衡偏聚实验观察

用硼径迹照相法和透射电镜观察了冷却过程中硼的晶界非平衡偏聚形成过程．实验发现,以2℃/s冷速从1150℃冷却到640℃的过程中,硼在晶界上的非平衡偏聚可以分成扩散行为不同的3个阶段．实验测定了晶界富集因子、富集带宽度、贫硼区宽度和贫化因子等偏聚特征,并对有关现象进行了初步讨论．     

铁素体中Ce与P晶界共偏聚的化学状态

用XPS结合计算机模拟研究了Ce与P在铁素体晶界上的偏聚状态。结果表明:当晶界P偏聚浓度较低,并以固溶态存在时,Ce对P的化学状态影响较小;当P的晶界浓度较高时,并以不同的化学状态存在时,Ce的影响较大,较多的Ce会改变P的化学状态。计算机模拟表明,晶界上P的化学状态取决于其偏聚结构。     

Effect of prior austenitic grain boundary composition on temper brittleness in a Ni-Cr-Sb steel

Grain boundary segregation during temper embrittlement of an Sb-containing, Ni-Cr steel has been examined both by Auger electron analysis and by chemical analysis by neutron activation of residues of surface layers dissolved by etching intercrystalline fracture surfaces. No grain boundary segregation of either alloying additions or impurities was detected during austenitization or tempering. Redistribution of Cr, Ni, and Sb between carbide and ferrite was observed during tempering, but no grain boundary segregation was noted. Both Ni and Sb were observed to segregate to the boundaries during embrittling. The segregated Sb was shown to be uniformly distributed along the prior austenitic grain boundaries and to control the ductile brittle transition temperature of the alloy studied. Ni segregating to the prior austenitic boundaries during embrittling was shown to be localized in a phase other than the ferritic portions of the boundaries. A possible location was shown to be the ferritecarbide interfaces in the grain boundaries. Weakening of these normally tenacious carbide and ferrite interfaces could account for the change in mode of brittle failure from transcrystalline cleavage to intercrystalline along the prior austenitic grain boundaries that is observed in temper brittle steels.     

Equilibrium grain-boundary segregation and the effect of boron in B-doped Fe−3wt%Ni austenitic alloy

The equilibrium segregation of boron at grain boundaries and the relation between grain boundary hardening and segregation in B-doped Fe−30 wt%Ni austenitic alloy were studied with the methods of microhardness measurement and Particle Tracking Autoradiography (PTA). It was found that equilibrium segregation of boron at grain boundaries appeared in the alloy as annealed between 650 and 960°C and no segregation of boron appeared above 1050°C. It could be concluded that an excess grain boundary hardening by addition of boron to the alloy was caused by the grain boundary segregation of boron.     

Austenitization during intercritical annealing of an Fe-C-Si-Mn dual-phase steel

Partial austenitization during the intercritical annealing of an Fe-2.2 pct Si-1.8 pct Mn-0.04 pct C steel has been investigated on four kinds of starting microstructures. It has been found that austenite formation during the annealing can be interpreted in terms of a carbon diffusion-limited growth process. The preferential growth of austenite along the ferrite grain boundaries was explained by the rapid carbon supply from the dissolving carbide particles to the growing fronts of austenite particles along the newly formed austenite grain boundaries on the prior ferrite grain boundaries. The preferential austenitization along the grain boundaries proceeded rapidly, but the austenite growth became slowed down after the ferrite grain boundaries were site-saturated with austenite particles. When the ferrite grain boundaries were site-saturated with austenite particles in a coarse-grained structure, the austenite particles grew by the mode of Widmanstätten side plate rather than by the normal growth mode of planar interface displacement.     

Optimizing the boron effect

The effects of boron content varying from 0.0001 to 0.0110 wt pct were studied to determine the optimum boron range for commercially desirable combinations of hardenability and notch toughness in 11/4 in. thick steel plate made from grade ASTM A514-J. Increasing boron content up to 0.0025 pct resulted in a gradually increasing boron hardenability factor which reached a maximum value slightly greater than 3 at about 0.0020 boron content. The increase in the boron hardenability factor was accompanied by a modest decrease in the CV₁₅ TT. An optimum value was realized at about 0.0020 pct boron. This behavior is consistent with established theory that small amounts of boron, not detectable by conventional means, concentrate at the prior austenite grain boundaries suppressing the ferrite reaction and thus improving hardenability. Increasing the boron content above 0.0025 pct resulted in a decrease in the boron hardenability factor to a value of about 2.0 when 0.0060 boron was added. No further lowering of the boron hardenability factor was experienced with boron additions as high as 0.0110 pct. Additions of boron above 0.0025 pct caused a strong increase in the CV₁₅ TT value. The deterioration of both the boron hardenability factor and toughness, when boron contents exceeded 0.0025 pct, was attributable to the formatipn of large particles of the “boron constituent,” identified as Fe₂₃(C, B)₆ which formed at the prior austenite grain boundaries and nucleated massive ferrite. The mechanism was operable at austenitizing temperatures and cooling rates equivalent to those used in commercial practice. This paper is based on a presentation made at a symposium on “Hardenability” held at the Cleveland Meeting of The Metallurgical Society of AIME, October 17, 1972, under the sponsorship of the IMD Heat Treatment Committee.     

Improvement of surface quality on peritectic steel slabs

Peritectic steel grades are very sensitive to microcracking along austenitic grain boundaries in continuous casting. Irsid and Aktiengesellschaft der Dillinger Hüttenwerke (DH) have combined laboratory studies and industrial trials to improve surface quality on these sensitive grades. Laboratory studies at Irsid confirmed the hypothesis that a very thin layer of ferrite along austenitic grain boundaries is detrimental for cracking and indicate that the risk of cracking decreases as soon as ferrite ratio is above 10 %. Dilatometric investigations demonstrate that there is a strong shift between thermodynamic equilibrium and beginning of γ→α phase transformation under casting conditions. Furthermore, at the slab surface, there is no cyclic transformation γ→α→γ induced by thermal cycling in front of spray nozzles and supporting rolls. DH performed trials with various cooling strategies on its new vertical caster No. 5. No cracks appear with intensive cooling whereas microcracks are present with soft cooling. These results are in agreement with laboratory studies. Intensive cooling is the standard condition at DH. With this process, microcracking is avoided for all slab formats.     

Overview no. 63 Non-equilibrium grain boundary segregation of boron in austenitic stainless steel-II. Fine scale segregation behaviour

A combination of TEM, FIM, AP and IAP has been used to study boron grain boundary segregation in austenitic stainless steels of the types 316L (with 40 ppm or with <1 ppm boron) and “Mo-free 316L” (23 ppm boron). High resolution segregation profiles were determined for cooling rates from 0.29 to 530°C/s for three starting temperatures: 800, 1075 and 1250°C. Boron grain boundary segregation was found after all heat treatments. The segregation behaviour was mainly of the nonequilibrium type after cooling from 1075 or 1250°C whereas equilibrium segregation dominated after rapid cooling from 800°C. The influence of the relative grain orientation on the amount of non-equilibrium segregation was small for general boundaries. However, no segregation was detected at coherent twin boundaries. The binding energy of boron to austenite grain boundaries was estimated at 0.65 ± 0.04 eV for both types of steels. The influence of the composition and boron content of the steels on the segregation behaviour is discussed and the experimental techniques used are presented.     

Phase stability and atom probe field ion microscopy of type 308 cre stainless steel weld metal

Improvement in high-temperature creep-rupture properties of type 308 stainless steel welds due to the controlled addition of boron is related to microstructural evolution during welding and thermal phase stability at creep service temperatures. The microstructure of boron-containing type 308 austenitic stainless steel welds, in the as-welded state, consisted of 8 to 10 pct ferrite in an austenite matrix. Atom probe field ion microscopy studies revealed segregation of boron and carbon to ferriteaustenite boundaries in the as-welded state; the segregation level was less than one monolayer coverage. On aging at 923 K for 100 hours, M₂₃C₆ carbides precipitated at ferrite-austenite boundaries. On further aging at 923 K for 1000 hours, the ferrite transformed into σ phase. Similar microstructural evolution was observed in a type 308 stainless steel weld without boron addition. The volume fractions of M₂₃C₆ carbides were identical in boron-containing and boron-free welds. Atom probe results from the welds with boron addition in the aged condition showed that the boron dissolved in the M₂₃C₆ carbides. However, lattice parameter analysis showed no apparent difference in the extracted carbides from the welds with and without boron. Creep property improvement due to boron addition could not be related to any change in the volume fraction of carbides. However, the results suggest that the incorporation of boron into M₂₃C₆ carbides may reduce the tendency for cavity formation along the M₂₃C₆ carbide-austenite boundaries and hence improve the resistance to creep fracture. The observed microstructural evolution in welds is consistent with thermodynamic calculations by THERMOCALC software.     

On the Role of Grain Boundary Serration in Simulated Weld Heat-Affected Zone Liquation of a Wrought Nickel-Based Superalloy

The effects of grain boundary serration on boron segregation and liquation cracking behavior in a simulated weld heat-affected zone (HAZ) of a wrought nickel-based superalloy 263 have been investigated. The serrated grain boundaries formed by the developed heat treatment were highly resistant to boron segregation; the serrated sample contained 41.6 pct grain boundaries resistant to boron enrichment as compared with 14.6 pct in the unserrated sample. During weld thermal cycle simulation, liquated grain boundaries enriched with boron were observed at the peak temperature higher than 1333 K (1060 °C) in both unserrated and serrated samples; however, serrated grain boundaries exhibited a higher resistance to liquation. The primary cause of liquation in this alloy was associated with the segregation of the melting point depressing element boron at grain boundaries. The hot ductility testing result indicated that the serrated grain boundaries showed a lower susceptibility to liquation cracking; the grain boundary serration led to an approximate 15 K decrease in the brittle temperature range. These results reflect closely a significant decrease in interfacial energy as well as a grain boundary configuration change by the serration.     

Control of annealing twins in type 316 austenitic stainless steel

The twinning frequency expressed as the number of coherent twin boundaries per grain has been investigated as a function of grain size in type 316 and 316L commercial austenitic stainless steels with different carbon contents as well as doped with 30ppm of boron, and subjected to various thermomechanical treatments. Experiments have established that the number of coherent twin boundaries per grain (TB/grain) for grain sizes larger than 0.1 mm, increases linearly with grain size according to the equation: TB/grain = A + K_td, where A and K_t are constants and d is the true volume grain diameter. In the ultrafine and fine grain size range from 1.5 to 100 μm the number of twins per grain is the lowest one and virtually independent of grain size. Both the increase in carbon content by 0.01% and doping with 30 ppm of boron, significantly suppress the formation of twins. Also, the rate of twins formation depends on whether twins are formed during pure grain growth or simultaneous recrystallization and grain growth. The results obtained are explained in terms of the classical Fullman-Fisher theory of twins formation.     

Local conditions at moving α/γ boundaries of proeutectoid ferrite transformation in iron alloys

The local conditions at moving α/γ boundaries in iron alloys are examined from the data on growth kinetics, solute partitioning, and critical limit of transformation. In Fe-C alloys, local equilibrium of carbon is likely to be sustained at the majority of α/γ boundaries during the growth of allotriomorphic ferrite except at some boundaries containing immobile low-energy facets. In Fe-C-X alloys, there is experimental evidence that local equilibrium of the substitutional alloying element is established at higher temperatures. However, growth under near paraequilibrium conditions may be prevalent at lower temperatures and at early growth stages. The diffusion of alloying elements in ferrite and along the austenite grain boundary may have a significant influence on the growth of ferrite near the boundary between fast and slow growth. The growth of Widmanstätten and bainitic ferrite is likely controlled by carbon diffusion, that is, without a supersaturation of carbon, while the chemical condition of carbon near the plate edge may not be identical to that of a planar disordered α/γ boundary.